1. Field of the Invention
The present invention relates to an electrophotographic image fixing device and particularly, to a color electrophotographic image fixing device.
2. Description of Related Art
A step of fixing a toner image comprising toner particles on a toner substrate such as a paper or a plastic film is essential in electrophotographic processing. Various methods are employed for the fixing.
As one of them, a heat-fixing method, which comprises heating a toner image to fuse and fix it on a toner substrate, is usually and widely employed. For this purpose, it is necessary to heat the toner image up to the temperature at which the component which constitutes the toner becomes adhesive. Thus, in many cases, the toner is softened on the surface of a paper or a plastic film, which is the substrate, and when cooled, the toner is solidified and firmly stuck onto the substrate.
A specific means of heating and fusion-depositing the toner in an electrophotographic processing apparatus is passing a toner substrate having a toner image on its surface between a pair of rollers consisting of a heating roller and a pressure roller. At least one of the rollers is provided with an external or internal heating means and is heated to a temperature higher than the softening point of the toner.
A roller comprising a mandrel coated with an organosiloxane rubber or polytetrafluoroethylene is known for use as the heating roller. The organosiloxane rubber roller is generally coated with a silicone oil for releasing purpose. However, after repeated use, releasability from the surface of the organosiloxane rubber roller decreases, and offsetting of the toner or the substrate is apt to occur. Especially, when a color image, the toner of which is readily fused and is 2-3 times larger in the amount on the substrate than a monochromatic image, is continuously fixed by the roller, releasability from the organosiloxane rubber roller conspicuously decreases, and offset occurs several times sooner than when a monochromatic image is fixed.
On the other hand, the polytetrafluoro-ethylene-coated roller is generally superior in its releasing performance and endurance. However, since the thickness of the coat is small, the roller is poor in elasticity and spreads the toner by the fixing pressure to cause deterioration in resolution of the resulting image.
Since the toner substrate is pressed onto the heating roller by the pressure roller, the toner substrate which has passed through the fixing device is pulled by the heating roller, and offset is apt to occur. In order to inhibit this, a separating nail is provided in contact with the surface of the heating roller on the toner substrate-discharging side of the roller. However, the separating nail sometimes scratches the surface of the heating roller or leaves stripes on the images, causing considerable reduction in the quality of images.
For improving releasability between the heating roller and the toner substrate, a releasing oil such as silicone oil is applied to the heating roller. In this case, however, a means for applying the oil must be provided and and furthermore, the oil tends to stain the toner substrate.
Especially, when full-color images are to be formed, three primary color toners of yellow, magenta and cyan are superimposed, and various colors are reproduced by the subtractive color mixture. Therefore, in this case, thickness of the toners deposited on the toner substrate is 2-3 times that of the toners in the case of the conventional monochromatic image. Thus, inhibition of occurrence of the offset becomes more difficult.
For avoiding occurrence of the offset of color images, a method has been proposed which comprises transferring to a transfer paper a toner image formed by electrophotographic process using, as the color particles of yellow, magenta and cyan, toners of particular resin particles prepared by mixing a pigment with a dye treated with a resin different from the binder resin, passing the transfer paper between a heating roller and a pressure roller having a rubber-like elastic material on the mandrel in a hot roller fixing device, and discharging the transfer paper therefrom with shifting to the pressure roller side from the vertical direction of the line connecting the center of the heating roller and that of the pressure roller [Japanese Patent Kokai (Laid-Open) No. 2-293867].